1. Field of the Invention
This invention relates to magnetic recording disks, and relates more particularly to such disks formed as a composite structure.
2. Description of the Prior Art
In magnetic recording, rigid record carriers are used which usually consist of an aluminum substrate and a magnetic layer provided thereon consisting of a dispersion of a magnetic pigment in a curable binder. The magnetic layer of record carriers of that type is usually between 0.5 and 1.5 .mu.m thick and can be used for record densities of up to 10,000 bpi. For higher record densities magnetic layers are required which differ, regarding the layer thickness as well as the magnetic values, from the conventional oxide dispersion layers. U.S. Pat. No. 3,704,211 shows that the record density depends on the layer thickness and the coercive force. For that reason it has been attempted to reduce the layer thickness by increasing the packing density and the coercive force. This is achieved e.g. by applying extremely thin ferromagnetic films which are usually made by sputtering, evaporation, or electroplating of metals of the eighth subgroup of the periodic system in a high vacuum on an aluminum substrate, or by applying ferromagnetic films of corresponding metal salt solutions in accordance with galvanic or chemical processes. Magnetic thin film disks made by applying a magnetic coating on an aluminum substrate in accordance with one of the above mentioned methods usually have magnetic layers which are thinner than those which can be made with magnetic particles dispersed in a curable binder. The disadvantage of these thin film disks is that frequently there are corrosion problems, mainly because of the reaction between the metal substrate, which for weight reasons is aluminum, and the thin film deposited thereon. In these disks, one or several protective layers have had to be provided between the substrate and the magnetic thin film layer, and over the thin film layer. It is obvious that the application of protective layers represents a considerable cost factor in the production of thin film disks. Furthermore, for magnetic disks with conventional oxide dispersion layers, as well as for those with magnetic thin films, a substrate with a much greater thickness than that of the magnetic layer was used, which considerably adds to the weight of a device containing several such disks.
A considerable problem in connection with the production of magnetic disks on the basis of thin metallic ferromagnetic films is the providing of suitable substrates. As the layer thicknesses of metallic ferromagnetic films are much below that of commercially available magnetic storage devices coated with a dispersion of .delta.-iron-III-oxide in a binder, very high demands are made on the substrate surface with respect to surface roughness, and the absence of scratches and holes. It is therefore desirable to achieve maximum surface perfection quality of the magnetic disk substrates.
In the past, magnetic disks generally used as a substrate aluminum disks whose surface had been made particularly smooth through mechanical processing steps as grinding, forming and lapping. Depending on the type of process, however, there frequently appear lapping scratches which are very difficult to remove in a subsequent grinding process. In connection with very thin films, however, minimum recesses in the substrate surface cause defects in the overlying magnetic coating, and on the other hand an over-intensive grinding or polishing will subsequently impact the planarity of the substrate. This, in turn, causes an undesired amplitude modulation, and a contacting between the magnetic head and disk if the information recorded is read with flying magnetic heads. Another problem connected with the production of perfect substrates suitable for metal films is the quality of the aluminum alloy used. Oxide inclusions and cavities in the surface of the aluminum disk can cause holes and recesses in processing, and additional defects when the magnetic disk is used. The methods for polishing substrate surfaces and the coating methods could be improved, but further improvements in the quality of the magnetic disks can be achieved only with nearly perfect substrate surfaces.
Prior art describes a number of composite magnetic disks. Examples are U.S. Pat. No. 3,681,225, U.S. Pat. No. 4,069,360 and U.S. Pat. No. 4,376,963.
U.S. Pat. No. 3,681,225 describes a magnetic disk where a magnetic layer is made by electro-deposition on a synthetic resinous core. U.S. Pat. No. 4,069,360 describes a magnetic recording element where on a disk made of an alloy a layer of a non-magnetic alloy is provided which covers the surface of the alloy disk. On this layer there is a thin film consisting of a magnetic metallic medium and covering a polished or finely processed surface of the alloy layer, an amorphous inorganic oxide layer covering the thin film surface, and a lubrication layer covering the oxide layer and being of such a nature that it adheres thereto. U.S. Pat. No. 4,376,963 describes a composite structure for magnetic recording, where on a core of polymeric material at least one silicon disk is provided which on its outer surface is coated with the magnetic recording material. The surface of the silicon disk is excellent with respect to flatness and smoothness, but the disadvantage of this disk is that its diameter is restricted to the diameters of approximately 15 cm maximum usual for silicon wafers.